Japanese Patent Laid-Open (Kokai) Nos. 54-49611, 56-32228, and 55-38294 suggest pressure-proof, relatively thin-walled metal-made cans particularly well suited for containing beer, carbonated soft drinks, etc., with capacities usually ranging from about one to 10 liters. These known metal cans are each comprised of upper and lower seamless can sections having open ends, one of which is reduced in diameter and which are fitted together via an adhesive layer. The subsequent bonding of the interfitting end portions of the can sections provides an annular lap seam.
Drawing is the usual technique employed for producing the upper and lower sections of such metal cans. They must be proof against the chemical attack of the contents. Thus the blanks to be drawn have corrosionproof coatings (e.g., phenol epoxy coating or organosol) applied on their surfaces which are to become the inner surfaces of completed cans. However, although the inner surfaces of the cans are thus protected against corrosion, the metal is left exposed to the contents at the edges of the underlapping end portions of the can sections as these edges are formed by severance. In order to assure sufficient proofness of the metal cans against corrosion by the contents, the underlapping edge as well as neighboring inside surface portions should be covered with an adhesive layer.
Such adhesive layers may be provided by the application of a coating of liquid adhesive, such as that in the form of slurry, or thermosetting adhesive, as disclosed in Japanese Patent Laid-Open (Kokai) No. 55-153629, or by the electrostatic coating using a powdered adhesive. A preferred method is to thermally fuse a length of thermoplastic tape onto the outer surface of the open end portion of one of the constituent sections of each can in such a way that part of the tape projects beyond the edge of the can section. Then the projecting tape portion is folded over and thermally fused onto the edge and inner surface of the can section. Then the other can section is engaged with the adhesivetreated can section and is heat sealed thereto. This method will provide an adhesive layer of constant thickness, free from entrapped air or other defects, between the lapping ends of the can sections.
In the practice of the above preferred method, the use of a forced stream of heated air may be contemplated for melting or softening the projecting portion of the thermoplastic tape in attaching it to the edge and neighboring inner surface portion of one of the can sections. The use of heated air is not recommended, however, because the tape might become unduly thin at its bent portions or might be torn, thus failing to perform the corrosionproofing function to the full. An alternative method readily conceived may be the use of a roll or brush, thereby to fold the projecting tape portion inwardly preparatory to thermally fusing it. This alternative method is also objectionable as the folding of the tape over the heated can section would cause the tape to become thin at its bent portions, or torn or wrinkled, or to entrap air. Here again, no satisfactory results would be obtained.
Accordingly, an object of the present invention is to provide a method of, and apparatus for, the manufacture of a metal can having an annular seam, such that the projecting portion of the plastic tape that has been thermally fused onto the outer surface of the open end portion of one of the can sections can be thermally fused onto the edge and adjacent inner surface portion of the can section without tears or wrinkles, without thinning at the bent portions, or without air entrapment.
Another object of the invention is to provide apparatus for the manufacture of a metal can having an annular seam, which comprises means for automatically and positively heat-sealing a length of plastic tape onto the outer surface of the open end portion of one of the can sections so as to leave part of the tape projecting therefrom, without giving rise to defects such as wrinkles or nonadhering portions.
The can section that has had the adhesive layer formed on its open end portion as above is then fitted in the open end of the other can section. Then heat is applied to melt the adhesive layer and so to fuse the open ends of the can sections to each other into an annular seam. In order to make this seam sufficiently airtight, the can sections should be heat sealed to each other under pressure.
For pressure application for the above purpose, Japanese Patent Laid-Open Nos. 56-32228 and 57-28643 propose to insert the open end of one can section into that of the other can section while the latter is heated and expanded in diameter, and then to allow the outer end to cool and shrink into tight fit with the inner end. This method requires an added step of thermally expanding the open end portion of one of the can sections. Another difficulty is the delicate temperature control necessitated to cause the can sections to fit together under a required degree of pressure.
According to another solution suggested by Japanese Patent Laid-Open No. 55-153629, the open end of one can section is forced into that of the other can section which is supported in a die. The subsequent springback of the inserted can section produces the pressure required for its tight fit with the other can section.
A problem arises in connection with this solution in the case where the can section to be pressfitted in the other is generally of thin wall. Particularly if the can section is made of material of relatively low rigidity, such as aluminum or its alloy, then its open end portion is easy to buckle circumferentially on reduction in diameter at the time of the forced insertion into the other can section. The buckling of the can section provides passages intercommunicating the interior and exterior of the completed can, to the detriment of the required air- and liquid-tightness of the seam.
It is therefore a further object of this invention to provide a method of, and apparatus for, the manufacture of a metal can having an annular seam, comprising a step of, and means for, interfitting the open ends of the can sections under sufficient pressure without the need for any such complex step as the thermal expansion of one can section or without the possibility of buckling the end of one can section engaged in that of the other.